A light emitting diode (LED) is a semiconductor element converting electric energy into light such as ultraviolet light and visible light, and has advantages of being long life and highly reliable, and when the LED is used as a light source, the time which exchanges lamps can be saved. An LED lamp in which an LED element is sealed by, for example, transparent resin is widely used in a backlight of a liquid crystal display device used in a display part of mobile communication equipment, PC peripheral equipment, OA equipment, household electric equipment or the like, and is also used in a lighting device such as a signal device, various switches, a lamp for automobile use, and illumination.
A color tone of light emitted from the LED lamp depends not only on an emission wavelength of the LED element, and light in a visible range from blue to red according to an intended use can be obtained by, for example, applying a phosphor on a surface of the LED element or including a phosphor in the transparent resin sealing the LED element. In particular, a white light emitting LED lamp is starting to be used for the backlight unit in the liquid crystal display device of the mobile communication equipment, the PC, or the like (See Reference 1).
When the backlight unit using the LED lamp is applied to the liquid crystal display device, light directivity is adjusted by simultaneously using an optical sheet of various types such as a optical diffusion film, a light guide plate, and a prism film. In other words, on a base substrate having a plurality of LED lamps mounted thereon in a plane state, there is overlaid the optical sheet of various types such as the optical diffusion film, the light guide plate, and the prism film, whereby the light directivity is adjusted. In a case as above, there is used a casing, for example, having engagement portions in a plurality of tiers on an inner wall, and the base substrate having the LED mounted thereon is caught by the engagement portion of a certain tier and the optical sheet is caught by the engagement portion of another tier, whereby both are housed in an overlaid manner in the casing (see Reference 2).
In a conventional backlight unit, a space exists between an optical sheet of various types and a base substrate having an LED lamp mounted thereon, in particular, between the optical sheet and the LED lamp. Since air exists in such a space as a matter of course, there is a problem that light emitted from the LED lamp is scattered by the air in the space, so that the light is not sufficiently transmitted to the optical sheet. This leads to a reduced surface luminance. To cope with the above problem, it is considered to shorten a distance between the optical sheet and the base substrate having the LED lamp mounted thereon to make the optical sheet contact the LED lamp. However, it is difficult as a matter of fact to make the both completely contact each other, and air exists, to no small extent, between the both.
Reference 3 describes a linear lighting device made by disposing a frame member around an LED element mounted on a base substrate in a bare chip state and filling light-transmitting sealing resin into the frame. Reference 4 describes a surface light emitting device in which a layer including at least one of an optical adhesive, an optical elastomer, and an optical gel is disposed between a light emission part and a light incident part of a light guide plate. If an optical material is simply disposed between the light emission part and the light guide plate, there is a possibility that light radiated from the light emission part based on a refractive index or the like of respective parts cannot be sufficiently transmitted to an optical sheet such as a light guide plate.
Further, since a temperature of the LED lamp rises to some extent at a time of light emission, coloring may occur depending on an optical material, reducing a light emission luminance. A light emission efficiency is decreased when the LED lamp reaches a high temperature. Therefore, it is necessary to cool the LED lamp at a time of lighting of the backlight unit, but a sufficient heat release structure has not been adopted so far.
Reference 1: JP-A 2003-160785
Reference 2: JP-A 2003-207780
Reference 3: JP-A 2004-165124
Reference 4: JP-A 2005-078802